Various image sensors are suitable for sensing radiation at different wavelengths, such as visible, ultraviolet (UV), deep UV (DUV), vacuum UV (VUV), extreme UV (EUV), and X-ray wavelengths, and also for sensing electrons. One class of image sensors performs time-domain integration (TDI). TDI image sensors are particularly suitable for use in dark-field inspection systems including those used to inspect photomasks, reticles, and semiconductor wafers. To detect small defects or particles on photomasks, reticles, and semiconductor wafers, low-noise image sensors are required. Sources of image-sensor noise include dark current within the image sensor, readout noise in the image sensor, noise in the “off-chip” electronics that amplify and digitize the sensor output signals, and noise from external electronics including drivers and controllers that couples into the signal. To meet high throughput and sensitivity requirements in dark-field defect-inspection systems, high vertical line rates, a multi-tap architecture, and an off-chip digitizer may be used. Also, vertical clock phases may have high voltage swings. Such systems are particularly demanding regarding noise.
In addition to noise, harmonics, spurious products, and other sources of interference degrade the performance of imaging systems. Such interference distorts weak signals. To ensure the image quality, signal distortion should be avoided or reduced as much as possible. The noise floor of an image sensor may be reduced by reducing or removing interference (e.g., from spurious signals) through the use of multi-sampling. Multi-sampling, however, lowers the horizontal clock rate and thus reduces throughput.
The image sensor and digitizer (or digital-signal-processing (DSP) chip) are packaged (e.g., wired or flip-chip bonded into a package). The total subsystem noise performance depends not only on the image-sensor design itself but also on the package design and the design of the camera's printed circuit board (PCB) electronics. To enhance dark noise-floor performance, the minimization of electrical crosstalk in various light levels is also important. While multi-tap sensor architectures provide increased throughput, such architectures conventionally employ multiple synchronous (i.e., multi-sync) clock domains. (A clock domain refers to circuitry that operates according to transitions of a particular clock signal.) The image signal is susceptible to the electrical crosstalk or interference due to multi-sync clock domains (e.g., including domains for horizontal-register, reset, vertical-register, signal, and reference clocks), which can couple into power and ground planes in a complex manner.
In sensor systems with a multi-tap sensor architecture, the ground and power planes may be continuous to provide signal integrity at high frequency, or may be split to eliminate global crosstalk. Splitting ground and power planes may trade global noise disturbances for localized noise disturbances, if the plane splitting does not fully dampen high-frequency ground currents. Such localized noise disturbances will degrade low-light-level performance of an image sensor.
Systems with large (e.g., continuous) ground and power planes may encounter excited resonance behavior while running multi-sync clocks. In this situation, global crosstalk or interference often appears on dark (i.e., unilluminated) pixels while other pixels are illuminated. This type of global crosstalk or interference will degrade (i.e., increase) the dark noise floor of the dark pixels, which are affected by the illuminated pixels in the dark-field inspection system.
The calibration process for reducing crosstalk, as performed during manufacturing, is typically a cumbersome trial-and-error procedure that attempts to minimize crosstalk for a specific image sensor. Such a procedure may involve setting the reset clocking (i.e., the timing for clock signals that bias reset gates) to be periodic toward an edge of the image sensor, with reset clocks being spread out.